Oceans May Be Speeding Melt of Greenland's Glaciers

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Dynamic layers of warm Atlantic and cold Arctic Ocean waters
around Greenland may be speeding the melt of the country's
glaciers, researchers find.

"Over the last 15 years or so, the
Greenland Ice Sheet has been putting a lot more ice into the
ocean," said Fiammetta Straneo of the Woods Hole Oceanographic
Institution, in Massachusetts, who has spent years studying the
ice-coated country that is currently responsible for about a
quarter of
worldwide sea level rise.

"We're trying to understand why, as we thought ice sheets changed
on much longer timescales, like thousands of years," she told
OurAmazingPlanet.

Researchers know that warm air over Greenland melts surface snow
and ice, but this process doesn't do enough melting to explain
the extent of the
glaciers' rapid retreat. The connection between ocean changes
— including a warming Atlantic Ocean — and glacier response is
"unchartered territory," and may make up the difference between
predictions of ice melt and reality, Straneo said.

Fjord flight

The edge of Helheim Glacier, a major outlet for the Greenland Ice
Sheet, is itself a largely unexplored region — and a case in
point. Between 2001 and 2005, the ice of the glacier retreated
about 5 miles (8 kilometers), with its continued shrinkage nearly
doubling in speed.

In August 2009 and March 2010, Straneo and her colleagues flew
helicopters over Sermilik Fjord, a 62-mile-long (100 kilometers),
5-mile-wide (8 kilometers) and up to half-mile-thick (900 meters)
chunk of ice extending into the ocean at the end of the glacier.

Based on samples from probes launched into rare patches of open
water near where the glacier meets the sea — an area extremely
difficult to reach by boat — the researchers discovered that the
interplay between glacier ice, freshwater runoff and water from
both oceans is far more complex than previously thought.

"We used to think that warm Atlantic water was just carried up,
melting the whole vertical face of the glacier," said Straneo.
"But we found that with this Arctic water sitting at surface, the
melting is really only at the bottom."

Little push, big change

Differences in water densities drive this dynamic, which appears
to be deforming the fjord and therefore affecting its stability
and the
flow of the glacier, according to the report published online
this week in the journal Nature Geoscience.

"We now think that small changes — increased warm water reaching
the edges, for example — may alter the shape of the glacier as it
plunges into the ocean, and this in turn affects the friction
that keeps the glacier from running fast," Straneo added. "A
little push is driving a big change."

The team also found that the circulation differs between the
summer and the winter. Yet current climate models take none of
these factors into account.

Sea level rise

Greenland holds enough water to raise sea levels 21 feet. While
such a catastrophic melt isn't expected anytime soon, short-term
impacts could still be severe as ocean waters continue to warm
with climate change.

Further, Straneo suggested the same complex interactions at this
fjord are likely playing out in other parts of Greenland, as well
as in Chile and Alaska. However, the
dynamics of Antarctica's glacier system are different, and
perhaps less susceptible to ocean temperature changes.

"This study shows that we really need to know more about how the
ocean changes at the edge of these glaciers before we can predict
changes to the glaciers," Straneo said. "As we interpret the past
or forecast future ice sheet variability and sea level rise, this
will greatly impact the answers that we get."